1. Field of the Invention
The invention relates to optical telecommunications and in particular to transmission systems in which binary optical signals are conveyed in the form of amplitude modulation of a carrier wave, the modulation is synchronized by a clock and the signals are organized into packets of binary information.
The invention relates to the problem of recovering from a received binary optical signal a clock signal comprising a stream of pulses having a stable recurrence frequency equal to that of the clock bit rhythm of the received signal.
2. Description of the Prior Art
Clock recovery is of particular benefit in regenerator devices for reshaping and resynchronizing the modulation of received optical signals which are affected by jitter after transmission in a network.
Prior art clock recovery devices initially used electronic circuits to process the optical signal after converting it into an electrical signal. All-optical devices have appeared more recently that supply an optical clock signal from an optical signal to the RZ modulation format, bypassing the electrical domain.
All-optical clock recovery devices include mode-locked lasers and self-pulsating lasers. One example of a self-pulsating laser that can be used as a clock recovery device was described at the OFC'2000 conference, Baltimore, Mar. 5–10, 2000, in a paper by S. Bauer et al entitled “Ultrafast Locking Optical Clock for IP Packet Switching Applications”, document TuF5, Mar. 7, 2000.
These all-optical clock recovery devices include a semiconductor optical amplifier medium inserted into a resonant cavity and into which is injected an electrical current slightly greater than a threshold current. This is known in the art. Laser oscillation can therefore become established, but leads to index variation and carrier density reduction in the medium, the effect of which is to interrupt the laser oscillation until a carrier density above the threshold is established again.
If no optical signal is injected into the amplifier medium, the device operates in a free oscillation mode. It emits pulses with one or more specific carrier wavelengths and having one or more specific repetition frequencies, as a function of the dimensions of the component and the magnitude of the injected current. If an optical wave amplitude-modulated at the rhythm of a clock having a frequency close to one of the specific repetition frequencies is injected into the amplifier medium, the device emits pulses at the rhythm of that clock.
Two characteristics of these clock recovery devices are the acquisition time and the holding time. The acquisition time Ta is the time for which the optical signal must be injected for the repetition frequency of the pulses to stabilize on the clock frequency of the signal. The holding time Tm is the time for which the repetition frequency remains stable after the signal disappears.
The acquisition and holding times depend firstly on respective binary sequences present at the start and at the end of the signal. The acquisition time is inversely proportional to the repetition rate of the pulses at the start of the signal (representing binary “1” in RZ modulation). The holding time is directly proportional to the pulse repetition rate at the end of the signal.
The acquisition and holding times also depend on the power of the injected signal: if the power increases, the acquisition and holding times decrease.
The above properties must be taken into account, especially in the case of clock recovery in asynchronous optical packet-switched transmission networks. This is because, in asynchronous optical packet-switched transmission networks, the packets formed at various sending points are received at switching nodes in which they are routed to transmission links corresponding to their respective destinations. Accordingly, the information takes the form of a succession of packets on the transmission links with diverse origins and whose clock rhythms have independent phases and frequencies that can be significantly different. Thus clock recovery must be effected at the rhythm of the received packets, which implies constraints on the acquisition and holding time of the clock recovery devices.
To define these constraints precisely, account is first taken of the fact that the switching nodes deliver to the regenerator devices packets comprising an unmodulated guard band followed by a modulated sequence called the payload. The payload starts with a preamble, continues with a sequence of information, and terminates with an end pattern. The guard band guarantees a low optical power during a particular minimum transmission time called the guard time Tg and aims to ensure a predefined minimum time-delay between sending on the same link of two successive payloads. Also, to facilitate clock recovery, the preamble and the end pattern comprise predefined sequences of successive “1” bits which, in the RZ format, correspond to successions of pulses at the rhythm of the signal bit clock.
The recovery device must then be in a position to supply a stable clock signal at the rhythm of the received packets and for a time period at least equal to the transmission time Tu of a payload. For this, a first condition is that the acquisition time Ta must be less than Tu, regardless of the sequence of information. In practice, this imposes that Ta must be less than the transmission time Te of the preamble.
Another condition is that the holding time Tm must be at least equal to the acquisition time Ta. However, given the existence of the end pattern, this condition is in practice always satisfied by the clock recovery devices previously mentioned.
Accordingly, the following conditions:Ta≦Tu,  (1)andTa≦Tm,  (2)
are in practice reduced to the following condition:Ta≦Te.  (3)
As already mentioned, increasing the average optical power of the injected signal reduces Ta and Tm. Accordingly, condition (3) can be satisfied by adjusting this average power to a sufficient level, and this has been verified experimentally.
However, injecting a signal at a sufficient power level into the device gives rise to another problem due to a phenomenon called the patterning effect. This effect, which is directly proportional to the power of the signal, becomes apparent when the injected signal includes long sequences of “1” or “0” bits, and is reflected in amplitude modulation of the clock pulses and high jitter during sequences of “0” bits. This phenomenon is represented schematically by timing diagrams a) and b) in FIG. 1, respectively showing amplitude variations as a function of time in a signal S injected into a self-pulsating laser or like device and a resulting clock signal CK. The signal S includes a first sequence comprising successive “1” bits and then a sequence comprising successive “0” bits and then another sequence comprising successive “1” bits, and it can be seen that the pulses forming the clock signal CK have a lower amplitude during the sequences of “1” bits and a frequency drift during the sequence of “0” bits.
This amplitude modulation and jitter make the regenerator devices less effective. This is because, as the clock signal is intended to be modulated as a function of the modulation of the signal to be regenerated to constitute the regenerated signal, it is important for the clock signal to have the most stable possible amplitude. Similarly, because the clock rhythm of the regenerator signal is that of the recovered clock signal, the latter must be free of jitter.
The amplitude modulation of the clock signal could be eliminated by means of an all-optical equalizer device of a type known in the art, but an all-optical equalizer device is incapable of eliminating the jitter.
Thus an object of the invention is to provide a solution to the problem previously stated that is effective and provides a short acquisition time Ta, which can be less than the transmission time Te of the payload preamble, whilst reducing the patterning effect mentioned above.